San Francisco Bicycle Coalition executive director Leah Shahum spoke at the ribbon-cutting and opening of Polk Street contraflow bikeway in May 2014.

One of the strongest San Francisco voices in advocating on behalf of bicyclists is stepping down from her role as head of the San Francisco Bicycle Coalition and heading to Europe.

Leah Shahum, often seen riding around town on her orange two-wheeler, announced in a letter to the nonprofit's members Tuesday that after 17 years with the group, 12 of which as its executive director, she is calling it quits by the year's end.

"It's time for me to pedal toward new adventures, including participating in a German Marshall Fund Fellowship to research the successes of Vision Zero in Europe," Shahum said in the letter.

In San Francisco, bike advocacy work can be something like a blood sport as the conflicts between drivers and bicyclists play out in policy debates between the mayor, Board of Supervisors and San Francisco Municipal Transportation Agency, which provides funding to the coalition. In recent months she was part of a coalition to adopt a Vision Zero plan, which sets policy in place to achieve zero bike and pedestrian fatalities by 2024. She also attempted to defy the mayor on his successful attempt to eliminate enforcement of Sunday parking meters.

For some, the mere mention of Shahum causes them to bristle. One of her most outspoken critics, Rob Anderson, who sued The City over plans to install bike lanes, said that Shahum "has simply been the leader of a special-interest group that has pushed its agenda regardless of the facts or reality."

"She and her organization have done a lot of damage to the city by pushing to redesign city streets on behalf of that small minority against the interests of everyone else that uses city streets," he said.

In March, Shahum targeted Board of Supervisors President David Chiu for siding with business owners in watering down a plan to install separated bike lanes on either side of the Polk Street.

SFMTA Transportation Director Ed Reiskin said Shahum's leadership and work with diverse communities "have helped our agency expand San Francisco's transportation options and design better streets with all modes of travel in mind."

As she steps away from her post, Shahum said the biggest threat to the bicyclists' movement is if city street changes are treated like a zero-sum gain.

"When the streets are made more accessible by adding physically separated bikeways and traffic calming measures, all road users benefit because we're all safer," Shahum said. "Our city leaders will need to show real backbone to stand up for the goal of Vision Zero."